An uninterruptible power supply, well known as a UPS, is typically used as a buffer between commercially supplied power and an electrical load, such as a computer, that requires precise or uninterrupted power. If line voltage varies or is interrupted, power to the load is maintained for a limited amount of time to operate the load during the interruption or to provide time to shut down the load without resulting damage. A UPS typically comprises a rectifier, inverter, and battery charger. The UPS uses the commercially supplied AC power to charge a DC battery. The computer or electronics equipment, the load, then draws its AC power from the DC battery through the DC to AC inverter using appropriate circuitry. This isolates the load from power surges or brownouts and also furnishes a source of power during brief outages.
There is an ever increasing need to protect consumer electronic products such as the computer and home entertainment equipment. Further, there is a need to integrate the UPS within the packaging style of product, and provide operational information to the consumer during normal and interrupted power operations for the consumer equipment relying on protection through the UPS.
With increasingly smaller and less expensive computer equipment, there has been an increasing emphasis on the need for smaller and less expensive UPS systems. Typically, small UPS systems are included in one of three categories; stand-by, on-line, and line interactive.
A stand-by UPS maintains its battery with a low charge rate, and only becomes active when the power source fails. Normally, power is passed from the alternating-current power source to the UPS output. Surge suppression capability is similar to a suppression power strip. All high power components are normally off, and are only used for a few minutes when the power source fails. With such an arrangement, heat build-up in the UPS is seldom a concern. On the other hand, since the UPS circuitry is only used when the power fails, a failure in the backup supply may not be noticed until backup power is needed, thus defeating the primary purpose of the UPS.
An on-line UPS converts the alternating-current power source to direct-current at the battery level, then regenerates alternating-current from the battery supply. Since the output power is being generated locally, the power source is effectively de-coupled from the UPS output, thus resulting in excellent surge suppression. Since the power generation circuitry is normally active, any failure in that circuitry is most likely to happen when the power source or main power is available. The UPS can switch back to the main power and the user can be notified that backup is not available before it is needed. Thus, there is no false sense of security as in the stand-by type systems. However, the on-line system requires a battery charge circuit with enough power to charge the battery and supply the alternating-current output under full load. Such a feature adds cost for providing the UPS system. Further, with high power components running continuously, heat build-up becomes a factor.
The line-interactive UPS is a stand-by supply that charges the battery by running alternating-current generation circuitry in reverse. It usually includes an extra winding on a power transformer that can be used to boost or cut the input voltage to keep the output voltage within a nominal range during power surges and brownouts. The line-interactive is currently popular in the small UPS market, very likely because of the lower number of component parts required for the UPS. Typically, all UPS systems must generate a sixty cycle alternating-current output, and consequently will have a relatively heavy, bulky transformer in order to operate at this frequency.
There is thus a need to provide a reliable UPS that fits within the consumer product lines including computers and home entertainment systems, and to provide such a UPS at a price that complements the products being protected.